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    <!-- 笨阶乘 -->
    <script>
      var clumsy = function (n) {
        let res = cal(n)
        for (n -= 3; n > 0; n -= 3) {
          res += n--
          res -= cal(n)
        }

        return res
      }

      // 计算一组 a*b/c，返回结果
      function cal(a) {
        let res = a
        if (--a > 0) res *= a
        if (--a > 0) res = Math.floor(res / a)
        return res
      }
      console.log(clumsy(4))
    </script>
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